For tolls, I think that for every turn a caravan is in someone else's territory, part of its income is "taxed" The player who owns it doesn't make quite as much, and the player whose territory it's in gains a little income. Although this might encourage really short caravan routes that only go to border towns/cities, so maybe it isn't the best idea...
That may or may not be relevant to Elemental, though (and I'm guessing no). I don't really see anything like GC2 trade showing up in Elemental - in other words, I don't think we'll be establishing random generic trade routes that will just generate income. I think trade routes in this game will be more functional - as in trading actual resources. Thus there isn't necessarily any 'profit' involved, and I wouldn't want to lose a percentage of my resources to kingdoms my caravans pass through. If there is GC2-style trading, though, then tolls would work. Otherwise I'd rather there be no tolls, just income generated when foreign caravans pass through your territory.
And how about, caravan spread influence/border? And if the caravan has not travel that area for long time, the border dwindles? After all, caravan means trade route & trade route spread influences.
Caravans extending your borders would be awesome. However, it might be worthwhile to have caravans be really good at expanding your borders in wild space, but not so quickly in claimed territory. I don't really want major traders to slowly erode everyone's borders with profuse amounts of caravans... Or maybe having a military presence in the area can greatly curb border-expansion from caravans?
Another reason of the “Conflict” state is that nowadays there are many countries that have border skirmishes that aren’t a full fledge war yet. I won’t consider that artificial.
In the real world, you don't need to be in conflict with another state for X number of turns before you're allowed to declare full fledged war 
Location specific RoP may be a bit difficult to do, in terms of UI. Although I like location specific RoP, I don’t mind there isn’t one. RoP is a very flexible way to deal with border.
I'm actually hoping to be able to draw routes on the map. For example, when making roads I'd like to be able to draw the route ahead of time and have my workers get to it. Another area where this could be handy is in custom trade routes (if the AI-picked route isn't satisfactory to you, or you want to do something strange). The ability to draw routes seems like a fairly simple way of giving the player a lot of effortless control in many areas of the game. Whether or not the AI could be made to consider such routes in a diplomatic treaty is where a location-specific RoP treaty might be difficult to implement.
I have skim thru that huge international trade thread. My probably inaccurate understanding about it is that international merchant guild escort all caravans. The more you pay them, better the troops are used to escort your caravans; and they also pay the sending and receiving party money (as insurance payout) if the caravan is destroyed during transit. I also like your idea of “foreign caravans passing through a kingdom's territory to generate income for that kingdom, but I don't want the originators of the caravans to have to pay it”. How about the international merchant guild pays the toll for every caravan that walks your land? It'll be a portion of that escorting fee they receive.
Mind you the guild still obeys all RoP btw countries on behalf its customers. If SD devs really fancy RoP or merchant guilds, I will definitely like them to smuggle stuff that violate RoP, if you pay them enough bribe. Alas... this might be too off-topic.
This is a it off topic and the issue of borders deserves its own thread. That said, I can't help myself... I'd rather a 3rd Party Merchant's Guild in charge of international trade not have to deal with RoP (too much micromanagement). On the topic of what route international trade caravans take, refer to that thread if you dare
Actually, GC2 spheres with a diplo-optional "get out" button and a few other features sounds like a pretty good idea.
I would prefer something more like Climber's OP and my replies, but I would be satisfied with this as well. This is somewhat off topic, but one thing Elemental definitely needs is the ability to threaten other players through diplomacy - and not just about getting off your land.
